*Swaggers in with experience*
For the Western RP I'd recommend a lyncher. Essentially, you establish some sort of area that the players would gather in (city, bar, prison, whatever) and allow them to mingle and sort out the backstory for a day or two. Then after a couple days of getting into it, you kill someone off in a plot event(choose randomly or whoever you think will move the plot most by dying).
The hardest part is writing a plot for the RP. To be honest, I never had anything in mind until the first few deaths and lynches were done, thus explaining the "leisurely" pace at which I updated. I recommend just pulling from literary influences until you've got a decent enough and believable plot. (My zombie RP took a turn about halfway through once I started reading The Stand)
Once someone's been murdered, you let the survivors vote on who is the perp. The trick is to leave just enough clues in the killings so you can say "It was __________ all along!" at the end but don't give things away. The actual killer is PM'd by the GM prior to the kick off and either selected at random (makes things hard for writing plot), or whoever you think would work best with the plot, just be careful not to make it too obvious or else people get it within days.
When the players have voted, then you post a lynch. This is just a scene in which some of the players who did the voting kill the accused in a manner that suits their character. This cycle continues until you decide to have a finale in which there are only a few people left (I liked to do 3-4). If the players guess the killer before the finale, then just wrap the story up with everyone moseying off into the sunset or something. If they do not guess the killer before the group shrinks to whatever predetermined amount you're aiming for, then either everyone dies or those with particularly devious characters talk their way out of death and thus the plot is wrapped up.
I hope this helps anyone who wants to GM. It's pretty fun once you get into it.
